Signal synthesizers are often used as local oscillators in the frequency conversion stages of high-frequency spectrum analyzers. The frequency conversion stages translate the frequencies of applied input signals so that the spectral characteristics of the input signals can be analyzed and represented by the spectrum analyzer on a display or other output device. As shown in FIG. 1, an input signal SIN applied to a first frequency conversion stage FCS1 is converted to a first intermediate frequency signal by mixing the input signal SIN with a signal provided by the first local oscillator LO1 in the first frequency conversion stage. The first intermediate frequency signal IF1 is converted to intermediate frequency signals IF2, IF3, having successively lower frequencies, by frequency conversion stages FCS2, FCS3, respectively.
For the spectrum analyzer to accurately represent the input signal SIN, the phase noise of the local oscillators LO1-LO3 should not contribute significantly to the phase noise of the resulting frequency-translated input signal at the intermediate frequencies. Accordingly, there is motivation to minimize the phase noise contributed by the signal synthesizers that are used as local oscillators in the spectrum analyzer.